When, therefore, a plate of silver, communicating with the leg of a frog, was laid upon glass, and a plate of zinc was lowered horizontally upon it, the capacities of both, for any electricity which they might have contained, must have been so much increased, that no one will suspect the contractions of the frog’s leg, to have been occasioned by any discharge of the electrical fluid from them. As little are we authorised to suppose, that the contractions were produced in consequence of the metals attracting the electrical fluid from the leg: for, since the leg was insulated, it is impossible that it should have received a new supply of electricity, after having been deprived by the metals of what it naturally possessed; and consequently, after once or twice contracting, no further contractions should have taken place: but this is contrary to the fact. I have before shewn, that flakes of gold leaf, placed between the metals, were not affected by their approach to each other; and that, besides, a quantity of electricity, sufficient strongly to affect an electrometer, was far too weak to excite contractions in the muscles of a frog. That this influence, however, whatever it be, is not derived from the metals alone, but that animals at least contribute to its production, as well as indicate its presence, is, I think, rendered highly probable, by what I have already urged, relative to the necessity of a communication between the metals, and the muscles, as well as between the metals and the nerves. I may likewise observe, that animals appear to have a much more complete controul over its effects, than one would expect them to have over an influence wholely external to them. When living and entire frogs are placed upon a plate of zinc, or tin-foil, and a piece of silver, or of gold, is passed over different parts of their legs, and thighs, till it come into contact with the plate; contractions are very seldom produced, and scarcely ever, if the frogs be healthy and upon their guard. But the instant their sciatic nerves are divided, the contractions produced are as free and vigorous, as if the legs had been completely separated from the body. This difference is not owing to the silver coming in contact with the wound, necessarily made in order to divide the nerve; for I have always taken care that it should not, and indeed when it did, no contractions were produced, unless the nerve had been divided.